1. Field of the Invention
This invention relates to a thermal head for making a stencil by thermally perforating a heat-sensitive stencil material, and more particularly to an inexpensive thermal head formed by a thick film process.
2. Description of the Related Art
In stencil making apparatuses which have been put into practice, a heat-sensitive stencil material is used, and there have been known two stencil making systems. One of the stencil making systems is so-called a flash system in which an original having a printing area containing therein carbon is brought into a close contact with a heat-sensitive stencil material is perforated by heat when a printing area of the original is exposed through the stencil material to flashlight from a flash bulb, a xenon flashtube or the like. The other stencil making system is so-called a digital system in which a stencil material is thermally perforated by selectively energizing heater elements of a thermal head according to an image signal read out from an original through an image sensor or the like, or an image signal representing a document and/or an image created through a computer or the like. The digital system is now prevailing over the flash system since the digital system permits the document editing and the image processing. Though the thermal head was once a device exclusively used in facsimiles, thermal recording printers or the like, recently the thermal head has been modified so that it can be used in thermal stencil making. Recently, the modified thermal head has come to be used in a thermal stencil making apparatus of the digital system. As the stencil material, there have been known one comprising thermoplastic resin film laminated to a porous base sheet and one comprising thermoplastic resin film with no base sheet.
Specific structures of thermal heads to be used in thermal stencil making are disclosed, for instance, in the following patent publications.
In Japanese Unexamined Patent Publication Nos. 63(1988)-191654 and 6(1994)-191003, the thickness of the protective layer of the thermal head is defined. In Japanese Unexamined Patent Publication Nos. 2(1990)-67133, 4(1992)-71847, 4(1992)-265759, 5(1993)-345401, 5(1993)-345402, 5(1993)-345403 and 6(1994)-115042, the length in the main scanning direction of the heater element and/or the length in the sub-scanning direction of the same is defined for the pitch of the heater elements in each direction. In Japanese Unexamined Patent Publication Nos. 4(1992)-45936, 7(1995)-68807 and 7(1995)-171940, there is disclosed a thermal head in which the heater element is not rectangular in shape. In Japanese Unexamined Patent Publication Nos. 4(1992)-314552 and 8(1996)-142299, there is disclosed a thermal head in which a cooling member is disposed between each pair of adjacent heater elements. In Japanese Unexamined Patent Publication Nos. 4(1992)-369575 and 8(1996)-132584, the shape or thickness of the glaze layer is defined. Further, in Japanese Unexamined Patent Publication No. 5(1993)-185574, the ratio of the length in the main scanning direction to the length in the sub-scanning direction of the heater element is defined.
Though not clearly described in the above patent publications, the thermal heads disclosed in the above patent publications can be considered to be of a thin film type except those disclosed in Japanese Unexamined Patent Publication Nos. 5(1993)-345401, 5(1993)-345402 and 5(1993)-345403. Actually, at present almost all the thermal stencil making apparatuses using a thermal head use a thin film thermal head, and those using a thick film type thermal head are limited to those for a postcard, those which function also as a word processor printer and those which function also as a heat transfer labeler. Only a very small fraction of the digital system thermal stencil making apparatuses uses a thick film type thermal head.
As pointed out by many of the aforesaid patent publications, it is preferred that the thermoplastic resin film of the stencil material be perforated in such a manner that perforations are discrete and adjacent perforations are not connected to each other. This is because of inherent characteristics of stencil printing that ink is viscous fluid and spreads wider than the area of the perforations when transferred to the printing paper through perforations of the stencil, and when the perforations are connected, the amount of ink transferred to the printing paper and the thickness of printed ink layer on the printing paper are acceleratedly increased and offset is caused. The thermal head for thermal stencil making differs in this point from that for thermal recording in which that recorded pixels overlaps each other is preferred.
In the digital system thermal stencil making, it is preferred that the perforations be separated from each other, the proportion of open area (the proportion of the area of the perforations per unit area of the thermoplastic film of the stencil material) be in a predetermined range (generally about 30 to 40% though depending upon the viscosity of the ink, the kind of the printing paper, the pressure at which the stencil is pressed against the printing paper, and the like) in order to ensure a proper printing density, and the shapes and the areas of the perforations be substantially uniform so that the unperforated portions between the perforations are arranged in a regular pattern and the densities of large printing areas such as solid parts are uniformed.
Typically, the thin film thermal head comprises a heat radiating plate of metal, an electrical insulating substrate and a glaze layer formed on the heat radiating plate in this order, a plurality of strip-like resistance heaters which are formed on the glaze layer to extend in one direction (the sub-scanning direction) and are arranged in a direction transverse to said one direction (the main scanning direction), and a plurality of electrodes each superposed on one of the strip-like resistance heaters with a part of the resistance heater exposed through a gap formed in the electrode. The exposed part of each strip-like resistance heater forms a heater element. That is, a pair of electrodes are formed on each resistance heater with their inner ends opposed to each other in the sub-scanning direction with a gap between. One of the electrodes is connected to a switching element for discretely energizing the heater element and the other electrode are integrated with the corresponding electrodes for the other heater elements to form a common electrode. When producing such a thin film thermal head, an electrical insulating substrate and a glaze layer are superposed on a heat radiating plate and a solid resistance heater layer and a solid electrode layer are formed in this order on the glaze layer. Then the electrode layer is removed along a line extending in the main scanning direction, thereby exposing the resistance heater layer in a line extending in the main scanning direction, and the resistance heater layer and the electrode layer are both removed in the sub-scanning direction at regular intervals in the main scanning direction. Thus, a plurality of strip-like resistance heater layers are formed each covered with a pair of electrode layers opposed to each other in the sub-scanning direction with a gap between. One of the electrode layer is connected to a switching element and forms a discrete electrode for discretely energizing the part of the resistance heater layer free from the electrode layer. The other electrode layer is integrated with the corresponding electrode layer for the other strip-like resistance heater layers to form a common electrode. A protective layer is formed to cover the discrete electrodes, the exposed part of the resistance heater layer and the common electrode. When an electric potential different from the common electrode is applied to a discrete electrode, the exposed part of each of the strip-like resistance heaters between the discrete electrode and the common electrode is energized and generates heat. That is, the exposed part of each of the strip-like resistance heaters between the discrete electrode and the common electrode forms a heater element.
Since the thin film thermal head is generally very small in heat capacity as compared with the thick film thermal head and the heater elements are separately independent of each other, the temperature distribution on the thermal head during operation is clear and the temperature difference between the high-temperature part and the low-temperature part (will be referred to as "the temperature contrast", hereinbelow) is large, whereby the thermoplastic resin film of the stencil material can be perforated in relatively uniform shapes according to the clear pattern of the temperature distribution. For this reason, in almost all of high-quality stencil making apparatuses, a thin film thermal head has been employed.
In the thermal recording, thick film thermal heads have been much employed as well as the thin film thermal heads. Typically, the thick film thermal head comprises a heat radiating plate of metal, an electrical insulating substrate and a glaze layer formed on the heat radiating plate in this order, discrete electrodes and common electrodes which are formed on the glaze layer alternately in the main scanning direction to extend in opposite directions in the sub-scanning direction with their inner end portions overlapping with each other in the main scanning direction, a strip-like resistance heater formed over the discrete electrodes and the common electrodes to extend in the main scanning direction across the discrete electrodes and the common electrodes, and a protective layer formed to cover the discrete electrodes, the common electrodes and the strip-like resistance heater.
When an electric potential different from the common electrode is applied to a discrete electrode, the parts of the strip-like resistance heater between the discrete electrode and two common electrodes on opposite sides of the discrete electrode are energized and generate heat. Each of the parts between the discrete electrodes and the common electrodes forms one heater element. However since on and off of the heater elements on opposite sides of a discrete electrode cannot be controlled independently of each other, that is, when one discrete electrode is applied with an electric potential, both the heater elements generate heat, and when one discrete electrode is not applied with an electric potential, none of the heater elements generate heat, the two heater elements should be considered to correspond to one pixel. The recording using such a thermal head will be referred to as "twin-dot recording", hereinbelow. When first common electrodes and second common electrodes of different lines are alternately disposed in place of the common electrodes so that the first and second common electrodes are electrically connected with one discrete electrode at different timings, on and off of the heater elements on opposite sides of a discrete electrode can be controlled independently of each other. In this case, one heater element corresponds to one pixel. The recording using such a thermal head will be referred to as "single-dot recording", hereinbelow.
In Japanese Unexamined Patent Publication Nos. 5(1993)-345401, 5(1993)-345402 and 5(1993)-345403, there is disclosed a thick film thermal head in which the lengths in the main and sub-scanning directions of each heater element (corresponding to one pixel) are smaller than scanning pitches in the main and sub-scanning directions, respectively, and the ratio of the length of the heater element in the main scanning direction to the main scanning pitch is substantially equal to the ratio of the length of the heater element in the sub-scanning direction to the sub-scanning pitch. The patent publications also say that the lengths in the main and sub-scanning directions of each heater element are equal to the diameters of a perforation in the main and sub-scanning directions, respectively. However, a stencil making apparatus using such a thick film thermal head has not been in wide use due to a problem in performance to be described later.
As can be understood from the description above, presently, substantially all the thermal stencil making apparatuses use the thin film thermal head.
The thick film thermal head is advantageous over the thin film thermal head by the following reasons: First, the productive facilities for the thick film thermal head is simpler and easier to manage than that for the thin film thermal head and accordingly, the thick film thermal head can be produced at lower cost. Second, unlike the thin film thermal head, the thick film thermal head can be produced in an open atomosphere without using, for instance, a sputter chamber in which the thermal head is to be confined, and accordingly, the thick film thermal head can be easily produced long. Accordingly, there has been demand for using the thick film thermal head in thermally making a stencil.
However, when the conventional thick film thermal head is used in thermal stencil making as it is, there arises a problem that printings made by the use of a stencil made by the thick film thermal head become lower in image quality. That is, as described above, the thick film thermal head is low in the temperature contrast as compared with the thin film thermal head, that is, the thick film thermal head is small in the temperature gradient as compared with the thin film thermal head. Since the resistance heater of the thick film thermal head is continuous in the main scanning direction, heat generated by each heater element is easily propagated in the main scanning direction. Accordingly, in the thick film thermal head, the temperature contrast in the main scanning direction is lower than in the thin film thermal head. Further, the thick film thermal head is larger in size of each heater element than the thin film thermal head. Especially in the thick film thermal head, the length in the sub-scanning direction of each heater element is generally about three times the scanning pitch in the sub-scanning direction, and accordingly, the temperature gradient in the sub-scanning direction at a given time is small. The volume of each heater element of the thick film thermal head is in the order of hundred times that of the thin film thermal head so long as they are equivalent to each other in resolution. Accordingly, the heater elements of the thick film thermal head is larger in heat capacity than those of the thin film thermal head, which results in slower temperature response to on and off of the applied pulses. This also corresponds to a low temperature contrast in the sub-scanning direction.
The shape of the perforations may be considered to basically correspond to the shape of areas where the experienced temperature on the thermoplastic film becomes not lower than a certain threshold value. However, actually, the temperature fluctuates from heater element to heater element, and the shape of the perforations are more apt to be affected by fluctuation in the temperature of the heat elements as the temperature contrast on the heater element becomes lower. Accordingly, the thick film thermal head is larger than the thin film thermal head in fluctuation of the shape of the perforations. Large fluctuation of the shape of the perforations results in microscopic unevenness in printing density and deteriorates evaluation of image quality. Further, fluctuation in the shape of the perforations is apt to result in enlarged and/or connected perforations, which can result in offset as described above.
Further, a state where the lengths in the main and sub-scanning directions of each heater element are equal to the diameters of a perforation in the main and sub-scanning directions as mentioned in Japanese Unexamined Patent Publication Nos. 5(1993)-345401, 5(1993)-345402 and 5(1993)-345403 is a very special case. This is because, in the thick film thermal head, the resistance heater is semi-cylindrical in cross-section and is the thickest at the middle in the sub-scanning direction, and as the distance from the middle of the resistance heater increases, the surface of the resistance heater becomes remoter from the thermoplastic film of the stencil material and the heat transfer efficiency deteriorates. The resistance heater is about 3 to 20 .mu.m in thickness. Accordingly, the distance between the surface of the resistance heater and the film of the stencil material is about 3 to 20 .mu.m at the edges of the resistance heater. In practical setting, at the time when the temperature of the heater element is maximized, the temperature at the middle of the heater element is, for instance, 350 to 400.degree. C., whereas the temperature at edges of the heater element is only 200 to 250.degree. C., which is substantially equal to the melting point of the film. Accordingly, when the edges of the heater element is at a distance of, for instance, 10 .mu.m in the vertical direction (the direction substantially perpendicular to the surface of the heater element), the perforation in the film can be hardly enlarged to portions opposed to the edges of the heater element.
On the other hand, the resistance heater of the thick film thermal head is substantially uniform in thickness in a cross-section in the main scanning direction. Further since the resistance heater is continuous in the main scanning direction, heat generated by each heater element is easily propagated in the main scanning direction. When printing a solid printing area, adjacent heater elements generate heat simultaneously, and accordingly, the temperature of inter-element portions (portions between the heater elements) is lower than the temperature of the heater elements at the middle thereof (350 to 400.degree. C.) only by about 50.degree. C.
As described above, the temperature contrast of the thick film thermal head highly depends upon the direction. Under such conditions, in order to make the ratio of the length of the heater element in the main scanning direction to the main scanning pitch smaller than 1 and substantially equal to the ratio of the length of the heater element in the sub-scanning direction to the sub-scanning pitch and to make the lengths of the heater element in the main and sub-scanning directions equal to the diameters of the perforation in the respective directions, it is necessary that the heat shrinkage stress of the film is highly anisotropic, which is practically impossible.
As can be understood from the description above, use of a thick film thermal head in thermally making a stencil is practically difficult mainly for reasons of quality of the perforations though proposed in Japanese Unexamined Patent Publication Nos. 5(1993)-345401, 5(1993)-345402 and 5(1993)-345403.